1. Technical Field of the Invention
The present invention relates generally to a gas sensor control apparatus which controls an operation of a gas sensor which is equipped with a solid electrolyte layer and works to measure the concentration of gas, and more particularly to such a control apparatus designed to ensure the accuracy of measurement in a gas sensor.
2. Background Art
Limiting current oxygen sensors are used as air-fuel (A/F) ratio sensors designed to measure the concentration of oxygen (O2) contained in exhaust emissions of motor vehicle engines to determine an air-fuel ratio of a mixture charged into the engine. A typical one of such oxygen sensors is equipped with a sensor device made up of a solid electrolyte layer made of, for example, zirconia and a pair of electrodes affixed to the solid electrolyte layer. The measurement of concentration of oxygen is achieved by applying the voltage to the solid electrolyte layer through the electrodes to produce a flow of electrical current through the sensor device as a function of the concentration of oxygen and sampling the electrical current for determining the air-fuel ratio of the mixture.
Ensuring the accuracy of measuring the concentration of oxygen requires control of the voltage to be applied to the sensor device. A variety of techniques are proposed to control the voltage to be applied to the sensor device. For example, Japanese Patent First Publication No. 2004-251891 teaches a gas concentration measuring apparatus which is designed to control the voltage to be applied to the sensor device using an applying voltage characteristic, as defined in the form of a first-order line (will also be referred to as an applying voltage characteristic line below) and to measure the concentration of oxygen in a wide range using an electric current (will also be referred to as a sensor current below) flowing through the sensor device. The gas concentration measuring apparatus also works to change the applying voltage characteristic based on the width of a limiting current range (i.e., a flat range of an output of the sensor device) appearing per given level of the concentration of oxygen within a gas concentration measuring range.
Specifically, the gas concentration measuring apparatus works to change the inclination of the applying voltage characteristic line to bring the voltage to be applied to the sensor device into within the limiting current range (i.e., the flat range) at each of the levels of the concentration of oxygen. The width of the flat ranges of the applying voltage characteristic may, however, be different between sensors. This may not be eliminated only by correcting the inclination of the applying voltage characteristic line. The control of the voltage to be applied to the sensor device has sill left room for improvement.